Position locating rescue transceiver

ABSTRACT

A secure, portable position locating radio has a geolocation receiver providing local position and timing information from geolocation means (e.g. GPS satellites) and a local transceiver for sending local position information to a communication system (e.g., an airborne or orbiting transceiver). A crypto unit is provided between the receiver and local transceiver for encrypting the local position information prior to transmission. A data processor coupled to the local transceiver, receiver and crypto unit controls operation of the device, including storing local position information and separating signals broadcast by the communication system into those intended or not intended for the device. An external interface and display and a real-time clock (e.g., slaved to the GPS receiver) are also coupled to the data processor. Encryption keys and message abbreviation codes are conveniently loaded into the device from a demountable accessory unit at the interface, and stored therein.

This is a continuation of application Ser. No. 08/299,029, filed Aug.31, 1994, now U.S. Pat. No. 5,748,147, which is a continuation-in-partof application Ser. No. 08/103,177 filed Aug. 6, 1993, now abandoned,which is a continuation application of Ser. No. 07/845,903, filed Mar.4, 1992, now abandoned.

FIELD OF THE INVENTION

The present invention concerns an improved means and method for portableradios and, more particularly, position locating and reporting portableradios.

BACKGROUND OF THE INVENTION

There is an ongoing need for portable radios, especially hand-portableradios, that allow a user to communicate with an airborne or satellitetransceiver, where the communication includes information on thelocation of the user. While such radios have many uses, they areespecially important in emergency and search and rescue situations.

Emergency and search and rescue radio location systems are known in theart. In one prior art arrangement known as the SARSAT System, anemergency radio transmitter continuously broadcasts a beacon signal at406 MHz. The beacon signal is picked up and recorded by a SARSATsatellite moving in low earth orbit. Later on in its orbit, the SARSATsatellite passes over a command and control base station to which itdownloads the recorded signals picked up from continuous transmitter.

The Doppler shift of the recorded signal as it approached, crossed overand receded from the beacon transmitter is analyzed in the base stationand the zero Doppler time determined. By knowing the orbital location ofthe SARSAT satellite at the zero Doppler time, a line of position of thebeacon transmitter is found. After several passes an approximate slantrange and approximate position of the beacon transmitter on the earth'ssurface determined. This estimated position is then communicated to arescue unit which then commences a local area search.

This approach has a number of weaknesses well known in the art. Forexample, the position of the emergency beacon transmitter is known onlyapproximately, there is a substantial time lag between initiation of theemergency transmitter and the local position determination, and anyonecan listen in so that the emergency signal and Doppler information arenot secure.

In another prior art emergency rescue system, a mobile emergency radiosends a signal directly to a local rescue unit to allow the local rescueunit to home in on the position of the emergency radio. While such unitsare effective, they suffer from a number of limitations well known inthe art, as for example, the rescue unit must be within range of theradio (typically line of sight) before any indication of the presence ofradio can be obtained, the rescue unit must carry direction findingequipment in order to home in on the radio, and considerable time may berequired to locate the emergency radio, thereby prolonging the exposureof the rescue unit and the user of the radio to any unfriendly personnelwhich may be in the area.

In a third prior art emergency rescue system, a standard marine radiotransceiver is coupled to a LORAN or OMEGA or satellite geolocationreceiver provided with a speech synthesizer. When an emergency switch onthe system is activated, the normal transceiver functions are disabledand it transmits a synthesized speech "Mayday" voice call andsynthesized voice stating the latitude and longitude, on the emergencychannel. Other information about the vessel or emergency may also berendered in voice by the speech synthesizer and transmitted at the sametime. A provision is made so that the emergency transmitter can bedisabled remotely by a signal sent to the unit. A difficulty with thissystem is that it has very limited range, uses open channels that can bemonitored by anyone, announces the position of the emergency unit inplain language by voice, takes a substantial time to communicate theposition of the emergency unit and can be disabled by a remote signal.This makes the users especially vulnerable to any unfriendlies that maybe monitoring the emergency channel.

Thus, there continues to be a need for an improved radio suitable forcommunication with airborne or satellite transceivers or relays andwhich automatically provides accurate information on its local positionand which is, preferably, protected against undesirable eavesdropping.

SUMMARY OF THE INVENTION

A mobile position locating radio has, in its most general form, ageolocation receiver for providing local position and timing informationfrom geolocation means and a local transceiver coupled to thegeolocation receiver for sending local position information to aworld-wide communication system adapted to receive position location andother information from the radio. In a preferred embodiment, thegeolocation information is provided by one or more satellites and thetransceiver is adapted to communicate with a satellite communicationsystem. A crypto unit is desirably provided between the geolocationreceiver and the local transceiver for encrypting and decryptingmessages to and from the mobile radio, including the local positioninformation. A data processor is also desirably provided coupled to thereceiver, crypto unit and transceiver for controlling access to andoperation of the radio. The data processor desirably selects signalsbroadcast by world-wide communication system intended for the particularradio being used.

In a preferred embodiment there is also provided an external interfaceand display and a real-time clock coupled to the data processor. Cryptokeys and message abbreviation codes are conveniently loaded into thedevice from a demountable accessory unit which couples to the interface.The local transceiver desirably operates in a burst mode and transmitsduring narrow time windows calculated using the local real time clockoutput slaved to accurate time/frequency information derived from thegeolocation means.

In a further embodiment, the accurate time/frequency information is alsoused to correct for anticipated Doppler shift of signals broadcast bythe local transmitter via the satellite communication system to a remotebase station to reduce link frequency error and signal acquisition timeat the base station.

In a preferred embodiment the device is small enough to be hand-held andis especially useful as an emergency radio for persons in distress. Thedata encryption and burst transmission features prevents those notintended to receive the message from reading the location of thetransceiver and other information exchanged.

As used herein the words "base station", singular or plural, areintended to refer to a transceiver able to communicate with anothertransceiver or a receiver. The base station may be fixed or mobile andearth-based or airborne or in orbit. The word "satellite", singular orplural, is intended to refer to one or more objects, usually a relaytransceiver (but not limited thereto), in earth orbit. The satellite maybe moving with respect to the earth or geostationary. The word"receiver", singular or plural, is intended to refer to a device forreceiving electromagnetic radiation. The word "transceiver", singular orplural, is intended to refer to a device for receiving and transmittingelectromagnetic radiation. The word "geolocation" is intended to referto information useful in determining local position, and the words"geolocation satellites" or "geolocation means" are intended to refer totransmitters capable of providing geolocation information.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a radio system used for search and rescue accordingto the present invention;

FIG. 2 shows a simplified schematic block diagram of a position locatingradio according to a preferred embodiment of the present invention;

FIG. 3 shows a simplified schematic block diagram of the positionlocating radio of FIG. 2 in greater detail;

FIG. 4 shows a simplified schematic block diagram of a base stationsuitable for use with the position locating radio of the presentinvention;

FIG. 5 shows a flow chart depicting mobile radio signaling operation;

FIGS. 6-7 show mobile radio transmit and receive message protocolsaccording to a preferred embodiment of the present invention;

FIG. 8 shows a flow chart depicting base station signaling operation;

FIG. 9 shows mobile radio transmit message protocol 200 in the samemanner as FIG. 6 but according to a further embodiment of the presentinvention;

FIG. 10 is similar to FIG. 9 but showing further details;

FIG. 11 is analogous to FIG. 2, showing a further embodiment of presentinvention;

FIG. 12 is a flow chart illustrating a preferred sequence for theoperation of the rescue radio of the present invention, according to afurther embodiment;

FIG. 13 is a simplified flow chart illustrating how a highly abbreviatedmessage sent by the rescue radio of the present invention is handled ina monitoring base station, according to a further embodiment of thepresent invention;

FIG. 14 shows a signaling protocol, analogous to FIG. 4, for anacknowledgment message sent from a monitoring base station or rescuevehicle to the rescue radio of the present invention in response to themessage illustrated in FIGS. 9-10; and

FIG. 15 shows the timing sequence of a repeated combination of thesignals described in FIGS. 9 and 14, including guard bands.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention is described in terms of certain geolocation andcommunication satellite facilities, but this is merely for convenienceof explanation and not intended to be limiting. As those of skill in theart will appreciate based on the description herein, the invented methodand system are not limited to the particular geolocation andcommunication systems used for illustration and the present invention isnot intended to be limited thereto and applies to other geolocation andcommunication systems as well.

FIG. 1 illustrates radio system 20 suitable for use for search andrescue and other functions according to the present invention. System 20comprises mobile radio unit 22 which communicates through antenna 21 andlink 23 with satellite transceiver 24, which in turn communicates vialink 25 with base station 26 which in turn relays information via link27 to rescue unit 28.

The well known MILSAT satellite system is suitable for transceiver 24.This system functions as a bi-directional relay transceiver, receivingsignals from any source in its receive antenna footprint on one set ofUHF channels (e.g., 290 to 318 MHz) and retransmitting the receivedsignals on another set of UHF channels (e.g., 240 to 270 MHz) to anyreceiver in its transmit antenna footprint. Intermediate relays, e.g.,satellite-satellite or satellite-ground-satellite, may also be used.Such an arrangement is referred to in the art as a "bent pipe"communication channel. While MILSAT is suitable, other satellite orairborne transceivers may also be used, as for example but not limitedto the IRIDIUM™/.SM. satellite cellular communication system currentlybeing constructed by Motorola, Inc. In general, the satellitecommunication link is referred to as a "SATCOM" link.

Mobile radio 22 receives information from geolocation satellite (orsatellites) 30 via down-link 32. The well known Global PositioningSystem (GPS) satellites are a suitable source of geolocation informationand are preferred, but other geolocation means may also be used. Forexample, the IRIDIUM and GLONAS satellite systems also providegeolocation information.

GPS satellites transmit typically in L band (e.g., 1.227 and 1.575 GHz).To facilitate receipt of geolocation information from geolocation means30, separate antenna 33 is desirably provided on radio 22 to accommodatethe GPS frequencies.

Based on the geolocation information received from geolocation means 30,radio 22 calculates its local position in some convenient coordinatesystem, as for example but not limited to, latitude and longitude. Theaccuracy of the geolocation information received from GPS satellites issuch that the local position of radio 22 may be determined typically towithin about 25 to 100 meters. The accuracy of the position determinedfrom GPS may depend upon whether or not the GPS system is functioning inits high or low resolution mode and/or whether the receiver is capableof decoding the high resolution mode. In any case, the accuracy withwhich the local position can be determined is much greater than thatwhich would be obtained were geolocation system 30 not utilized. Radio22 then transmits this local position information over links 23, 25 viasatellite 24 to station 26. In a preferred embodiment, this positioninformation is encrypted prior to transmission by radio 22. GPSreceivers and means and methods for processing GPS signals to obtain anaccurate local position are well known in the art and GPS receivers arecommercially available from numerous sources.

In a preferred embodiment, station 26 sends an acknowledgment signalback to mobile unit 22 via links 25, 23 and satellite 24, indicatingthat it has received the local position information from unit 22.Station 26 then passes this local position information via link 27 (orlinks 25', 27') to rescue station or unit 28. Since rescue unit 28 nowhas a relatively precise local position of radio 22, it may proceeddirectly to that location without having to perform a wide area search.This greatly facilitates rescue of or contact with the individual (orgroup) using radio 22 and decreases the time during which the individual(or group) using radio 22 and/or the personnel in rescue unit 28 may beexposed to unfriendly action if, for example, rescue is being attemptedfrom hostile territory. While in the present example, the local positioninformation transmitted by radio 22 is desirably used for rescue, it mayalso be used by system 20 for other purposes. Radio link 29 betweenrescue unit 28 and radio 22 is customarily referred to as the "LOS"(line-of-sight) link, since it is often at a frequency that is suitedfor line of sight communication.

FIG. 2 shows a simplified schematic block diagram of position locatingradio 22 according to a preferred embodiment of the present invention.Radio 22 comprises geolocation receiver 34 which receives geolocationsignals from geolocation means 30 via down-link 32 and antenna 33 andwhich provides local position information as previously described.Receiver 34 is conveniently a GPS receiver however other receivers, asfor example, an IRIDIUM or GLONAS receiver may also be used.

Geolocation receiver 34 also provides accurate time/frequencyinformation based on signals received from the highly stable masterclocks/oscillators in geolocation system 30. As those of skill in theart will understand, accurate time and accurate frequency areinterrelated and one may be used to obtain the other.

Geolocation receiver 34 is coupled by bus 36 to data processor 38. Dataprocessor 38 is typically a microcomputer, as for example a TypeMC68HC11 microprocessor unit manufactured by Motorola, Inc., Phoenix,Ariz., but other microcomputer or computer or controller means wellknown in the art may also be used. Data processor 38 may containon-board memory (e.g. RAM, ROM, PROM, EPROM, etc.) or such memory may beprovided by memory 39 coupled to processor 38 by bus 40

Data processor 38 is coupled to real time clock 41 by bus 42, totransceiver 44 by bus 46, to user interface and display 48 by bus 49,and to external I/O 52 by bus 54. Real time clock 41 provides timing andfrequency control signals to data processor 38 and transceiver 44. Realtime clock 41 is conveniently slaved, i.e., calibrated or corrected,using high accuracy time/frequency information obtained by geolocationreceiver 34 from signals 32 broadcast by geolocation means 30. Thisaccurate time/frequency information is optionally coupled from receiver34 to clock 41 via line 43 or may be coupled via data processor 38 viabusses 36, 42. Thus, receiver 22 has internally a very precisetime/frequency capability which, as will be subsequently explained isdesirably used to improve overall system performance.

User interface and display 48 provides status information on the system,alarm indications, message receipt acknowledgments, and various messagesbeing transmitted and received, and controls for actuating radio 22.Interface 48 permits the user of radio 22 to input various messages orother information and to operate radio 22. For example, radio 22 may beset to transmit its local position at preset times, or may be set totransmit voice messages to rescue unit 28 or receive such or transmitother signals to satellite 24 and/or rescue unit 28 at the option of theuser. Those of skill in the art will understand how to construct suchuser interface and display depending upon the functions and messageindications desired to be provided.

External I/O (input/output) 52 conveniently provides an interface (e.g.,a "fill" connector") whereby an auxiliary control unit can be used toload security keys or ID codes or other information which is temporarilystored within radio 22 and which is not conveniently or desirably loadedthrough user interface 48. For example, the user need not know what IDor security key has been assigned to the unit which he or she is given.This interface also conveniently permits data to be loaded in digitalform. Those of skill in the art will understand how to construct such aninterface depending upon the particular type of data or keys or codesdesired to be input in this manner.

While data processor 38 can communicate with transceiver 44 via bus 46,it is preferable that data be passed to and from transceiver 44 via dataencryption/decryption ("CRYPTO") module 56. As used herein the word"encryption" is intended to include decryption and vice-versa. Cryptomodule 56 is functionally coupled between data processor 38 andtransmitter 44 so that data from data processor 38 passes over bus 58 tocrypto module 56 and vice versa. Crypto 56 is coupled to transceiver 44by links 60, 62. This allows geolocation data and other information andmessages being provided to data processor 38 and information andmessages generated within data processor 38 to be encrypted by module 56before being sent by transmitter 44 and then via antenna 21 to satellite24 and/or rescue unit 28. Similarly, messages received by transceiver 44may be fed directly to data processor 38 via bus 46 or passed throughmodule 56 for decryption. The use of crypto module 56 is desirable sinceit prevents unintended persons from listening in on the conversationwith or data transmission to or from radio 22. In particular, thegeolocation information is protected. Other types of information whichare also desirably protected are user status, keys, authentication codesor pass-words, and other information messages.

Transceiver 44 is preferably a channelized receiver, that is, atransceiver which operates over a frequency spectrum subdivided intonarrow bandwidth channels. The use of a channelized transceiver isdesirable since it permits multiple units to communicate at the sametime and in the same location without interference, each radio beingassigned to use a particular channel at a particular time. Suchassignments may be static or dynamic and may be user selected or beremotely controlled. Means and methods for channelized receivers arewell known in the art.

FIG. 3 shows a simplified schematic block diagram of position locatingradio 22 of FIG. 2 in greater detail. Geolocation signals are receivedby antenna 33 and passed to geolocation receiver 34 which passes thedecoded position information and accurate time information to dataprocessor 38 and clock 41, as previously described. Signals to and fromsatellite 24 via SATCOM link 23 and to and from rescue unit 28 via LOSlink 29, pass through antenna 21 to transceiver 44.

Describing now the receiver path of transceiver 44, signals arriving atantenna 21 are passed to transmit-receive (T/R) switch 72. When in theSATCOM mode, received signals from T/R switch 72 are passed via link 74to band pass filter 76 and low noise amplifier 78 to switch 80, or whenin the LOS receive mode, received signals are passed directly via link75 from T/R switch 72 to switch 80. The path via filter 76 and amplifier78 are preferred in the case of signals received from satellite 24(because they are generally relatively weak) and path 75 is generallypreferred for signals received from rescue unit 28 (because they aregenerally stronger). Switch 80 may be automatic, that is, switch to path75 when the signal strength rises above a predetermined level, or it maybe frequency selective, that is, switch to path 75 when the receivedfrequencies are in a particular band. Those of skill in the radio artswill understand how to implement such elements. The output of switch 80is sent to bandpass filter 82, amplifier 84 and to mixer 86 where signal85 received from antenna 21 is mixed with a local oscillator signal online 87 from frequency synthesizer 100.

Output signal on line 89 from mixer 86 is sent to bandpass filter 90,amplifier 92, filter 94, and IF amplifier and demodulator 98. IFamplifier and demodulator 98 is desirably of a variable bandwidth typeso that its transfer properties are alterable by a signal on line 99arriving from data processor 38 to provide wide bandwidth for LOSsignals and narrow bandwidth for SATCOM signals. Output 101 containingdemodulated frequency modulation information is provided to crypto unit56 and output 103 containing demodulated amplitude modulationinformation is provided to audio module 108. IF/DEMOD unit 98 alsopasses wideband information via output line 99 to data processor 38.This information typically includes distance measuring equipment (DME)information (e.g., signal arrival time) used for retransmittingparticular signals to the DME in rescue unit 28 so that it can calculatethe range to radio 22.

Various encryption/decryption arrangements and apparatus may be used forcrypto module 56 depending upon the desired encryption/decryptionalgorithm. Encryption and decryption means and methods are well known inthe art, and any suitable arrangement may be used. The crypto module maybe more or less complicated depending upon the degree of securitydesired. Where the required degree of security is not high, a simplecrypto algorithm such as a substitution or look-up table or frequency ordata inversion is often suitable. Where higher degrees of security aredesired, a more robust algorithm should be used. Those of skill in theart will understand how to provide whatever degree of data security theydesire. It is desirable that the crypto function be able to proceed inreal time so as to not unduly delay transmission and receipt of thedesired position information and messages.

The decrypted audio on output 107 is passed to audio amplifier and audiotransmit/receive module 108, and thence via output 109 to audioinput/output voice transducer 110, and vice versa. Typically, transducer110 performs a dual function as both speaker and microphone controlledby a press-to-talk switch (not shown). Alternatively, output data bus 58from crypto module 56 may instruct data processor to present data ondisplay 48 or to retrieve a predetermined message from memory 39 whichis then communicated to audio module 108 via path 123. Persons of skillin the art will understand how to implement such elements.

Crypto module 56 also communicates with data processor 38 over bus 58and with the FM modulator input of synthesizer 100 via path 113. Dataprocessor 38 controls the flow of information to and from crypto module56. Data processor 38 also controls the action of frequency synthesizer100 via path 117. Memory 39 is coupled to data processor 38 by bus 40.

Data processor 38 also provides on line 121 to biphase modulator 114, anoutgoing data stream intended to be transmitted by radio 22. The datastream on line 121 returned in response to a LOS signal desirably has aprecisely determined delay compared to the incoming LOS signal so thatit may be used by rescue unit 28 to obtain range information. Modulator114 receives carrier frequency signals on line 115 from frequencygenerator 100. Modulated output on line 119 from modulator 114 is passedto T/R switch 124 and from thence on line 125 (Transmit mode) to RFpower amplifier 126 and T/R switch 72, and from thence to antenna 21.Output 87 from modulator 114 (Receive mode) goes to receive channelmixer 86, as previously described. The lines controlling switches 72, 80and 124 have been omitted for clarity, but those of skill in the artwill understand that they are present and that their operation isconveniently controlled by data processor 38.

Frequency synthesizer 100 is desirably a voltage controlled oscillatorwith internal phase lock loops or other arrangements well known in theart for generating a variety of predetermined frequencies derived from astable master oscillator which is in turn calibrated by accurate timingor frequency signals from clock 41 and GPS receiver 34 via dataprocessor 38. Various frequencies produced by frequency synthesizer 100are used in transmit and receive modes. Not only does frequencysynthesizer 100 provide the frequencies needed to operate on the desiredtransmit and receive channels, but also provides any other frequenciesused internally by the transceiver for modulating or demodulating thevarious signals being processed. Since the output of frequencysynthesizer 100 is controlled by data processor 38, the production ofsuch frequencies for different purposes and at different times duringreceive and/or transmit functions is readily accomplished. Computercontrolled oscillators are well known in the art. Thereceiver/transmitter function including encryption of the position andmessage information, is controlled by data processor 38.

FIG. 4 shows a simplified schematic block diagram of base station 26suitable for use with the position locating radio of the presentinvention. Base station 26 comprises antenna 142, transceiver 144,channelized receiver and signal processor 146, crypto module 148,synthesizer 150, data processor 152 and computer system 154.

Computer system 154 is typically a personal computer system orequivalent containing, for example, a keyboard, central processor,memory for data and program storage, display and, optionally, a printer.A PS/2™ type system manufactured by IBM Corporation of Armonk, N.Y. or aMacintosh™ II type personal computer, manufactured by Apple Computer,Inc. of Santa Clara, Calif. is suitable, but other systems of equal orgreater computing power may also be used. The system should be equippedwith an RS232 serial interface.

A signal received by transceiver 144 through antenna 142 via satellitecommunication system 24 from emergency radio 22 is generally a narrowband signal but may initially fall anywhere within the satellitechannel. Channelized receiver and signal processor 146 determines thefrequency of the received signal and sends this information to dataprocessor 152 which in turn adjusts frequency synthesizer 150 so thatthe transceiver 144 locks up on the received signal, and so thattransceiver 144 will transmit back to radio 22 on the correct frequencyneeded by radio 22.

In the receive mode, receiver/processor 146 routes the cipher text datato crypto module 148 which decrypts the cipher-text data and passes theplain-text data to data processor 152 for further action. Data processor152 outputs data and messages to computer system 154 which functions asthe human interface of base station 26. Computer system 154 stores anddisplays the position information and messages and other desiredinformation about the sender. Computer system 154 may also initiate anacknowledgment response to radio 22 and notify rescue unit 28, eitherautomatically or manually or a combination thereof.

In the transmit mode, the operator inputs through computer system 154whatever messages or other commands or signals are desired to be sent toradio 22 or such messages may be generated automatically by computersystem 154. This information is transferred to data processor 152 whereit is assembled into the correct format for encryption and transmission,according to the protocols and keys selected by the system designer anduser.

Data processor 152 also commands synthesizer 150 to set the propertransmit channel frequency and to key transceiver 144 when crypto module148 routes encrypted data to transceiver 144 for transmission to radio22.

One of the functions performed by base station 26 is to provideautomatic acknowledgment to radio 22 that its initial position signalhas been received. Base station 26 may also command radio 22 to transmitonly on specified frequencies and/or only at particularly designatedtimes and/or for particular durations or a combination thereof. Thus,while radio 22 may send out an initial emergency signal with itsposition information on a general purpose "sign-in" or "listen-only" oremergency channel, radio 22 can be instructed by base station 26 toshift transmission and reception to other available channels so as toleave the "listen-only" or "sign-in" channel free. Further, base station26 can continue to send signals commanding radio 22 to further shiftchannels at any later time.

The operation of emergency radio 22 will be further understood byreference to the flow chart of FIG. 5 illustrating the use of radio 22to transmit emergency signals according to method 160. In block 162,geolocation receiver 34 decodes the geolocation information fromgeolocation means 30 to obtain accurate position and time/frequencyinformation. In block 164, the position information is combined withother status information about the radio or user, as for example, theuser identifier, any messages desired to be sent and error detectioncodes provided by block 166. The assembled data is sent to crypto module56 in block 168 and the encrypted data is then combined in block 170with other data or synchronization bits (see FIG. 6) obtained frommessage leader information block 169, to make up the message to betransmitted.

The frequency to be used for transmission is selected in block 172 andtransmission is accomplished in block 174. The initial message may betransmitted on a preset "sign-in" channel whose frequency is stored inradio 22, having been loaded through interface 52. Alternatively, radio22 may send the message on a channel whose frequency is determined inblock 176 based on instructions received from communication system 24while radio 22 is in a passive, listen-only mode prior to or in betweenemergency transmissions.

A further feature of radio 22 is the ability to listen to communicationsystem 24 and, knowing the frequencies that should be coming from basestation 26 and satellite 24, and comparing the received frequencies tothe expected frequencies using the accurate time information determinedin block 162, determine the Doppler shift of received signals in block178 and use that information to shift the frequency in block 172, shiftthe frequency of the signals to be transmitted in block 174. Thefrequency shift is desirably sufficient to compensate for the Dopplershift to be expected as the signal from radio 22 passes throughcommunication system 24 to base station 26 so that the signal arrives atstation 26 well centered in a predetermined channel. This has theadvantage of reducing the signal acquisition time of the receiver inbase station 26, thereby improving the performance of the whole system.

FIG. 6 illustrates the preferred signal format for signals from radio 22to base station 26 employing 288 bit signal bursts at a bit rate of 600bps. As shown in FIG. 6, the first 32 bits are used for bit synch sothat the communication receivers in base station 26 can synchronize onthe bit rate. The next 16 bits are a unique word whose function is toidentify the beginning of the encoded data stream. Combined, these arereferred to as the "leader", and have a total of 32+16=48 bits in thepreferred arrangement.

A preferred bit allocation of the 120 "message" bits is, 16 bits for auser ID, 64 bits for the position data, 24 bits for a coded message, andthe final 16 bits for an error detection check sum. In order to increasethe robustness of message communication, redundancy is desirably addedto the "message" data stream by use of a rate 1/2 coder. Thus, the 120bit "message" is desirably manipulated to provide 240 encoded bitscontaining the same information as the original 120 "message" bits. Ifsome of the encoded message bits are lost during transmission, themessage can still be recovered. This makes a total of 48+240=288 bits tobe transmitted. Those of skill in the art will understand that the"encoding" referred to here is for the purpose of providing redundancyand is not to be confused with encryption (described elsewhere) whosefunction is to render the transmitted data stream opaque to any observernot possessing the appropriate algorithm and key. Encryption may beperformed before or after the rate 1/2 coding.

By allocating 24 bits for the coded messages, a very large number (over16 million) of predetermined messages or message combinations can besent, each one represented by a particular bit combination. For example,using 8 bit bytes, each byte standing for one of 256 alphanumericcharacter or symbols, words and numbers can be spelled out or a largenumber of canned (pre-agreed) messages represented by particularletter/number combinations can be transmitted. Examples of simple twoletter codes conveying substantial information are, "NI"="I am notinjured.", "SW"="I am heading southwest.", "MU"="There are manyunfriendlies in the area.", and so forth.

Thus, substantial information can be sent back and forth between theuser of radio 22 and base station 26 or rescue unit 28 during a verysmall and brief data burst, since 288 bits at 600 bps only requires atransmit time of about 0.48 second. This is a significant advantage tousers who may be in hostile territory and who wish to keep theirpresence a secret from any unfriendly forces in the area. Theabove-described arrangement provides a system and method that is robustin terms of communication capability, location precision available tothe rescuers, ease of use, portability, geographic area of coverage(virtually world-wide), message security, and being difficult to locateby hostile forces using trilateration on signal transmitted by therescue radio.

FIG. 7 indicates a preferred signal protocol for the return message frombase station 26 to mobile radio 22. It is desirably about 160 bits long.There is no position data and only an 8 bit check sum is needed.Otherwise the message structure is similar with an uncoded 48 bit leaderproviding bit sync and a unique word, and the remaining 112 symbolsdistributed as shown and encoded using a rate 1/2 coder. At 600 bps,this message only requires 0.27 seconds transmit time. This is importantin situations where a large number of emergency users must be handled atthe same time. The encoded ID serve as a specific user address oridentifier. The unique word at the beginning of the messages, which canbe left in plain text if desired, can also serve as an address by whichradio 22 can determine whether a signal relayed through satellite 24 isintended for radio 22 or another similar unit without having to decrypteach signal.

FIG. 8 shows the steps involved in receive/acknowledgment response 180by base station 26, in a preferred embodiment. In block 182 the receiverdetermines the frequency of the incoming signal and locks on. In block184 the signal may be sent as a whole to block 186 for decryption or theencoded "message" portion of the signal (see FIG. 6 for the signalformat) separated from the leader portion and sent for decryption inblock 186. An optional automatic "message received" response may beselected via path 187 and block 192 and transmitted back to radio 22 inblock 196. The desired message format for messages sent by base station26 is shown in FIG. 7. Alternatively or in addition, the position ofradio 22 and any message (see FIG. 6) combined with the positioninformation is separated and stored in block 190. The acknowledgmentmessage (see FIG. 7) may be delayed and executed at that step asindicated by path 191, analogous to path 187.

In block 194 the position and message are passed to local rescue unit 28and an acknowledgment signal, if not already sent, may be generated asindicated by path 195. Computer 154 or a human monitor or other agencythen selects a response which is encrypted in block 192 and transmittedback to radio 22 in block 196. Any or all of the responses to radio 22may contain instructions as to the frequencies and/or time slots radio22 should use to listen for messages addressed to radio 22 and when andon what frequencies radio 22 should transmit further messages to station26, and what unique address is included in the message so that radio 22may separate its traffic from that addressed to other units.

FIG. 9 shows mobile radio transmit message protocol 200 in the samemanner as FIG. 6 but according to a further embodiment of the presentinvention. In the preferred arrangement of this further embodiment,message protocol 200 comprises radio ID portion 202 (e.g., 16 bits),POSITION & TIME DATA portion 204 (e.g., 52 bits), MESSAGE portion 206(e.g., 8 bits), CHECK-SUM portion 208 (e.g., 8 bits) and UNIQUE WORDportion 210 (e.g., 16 bits). Fewer or more bits can be used for any orall of the portions 202-210 of message 200, but the examples shown arepreferred because they provide a highly useful compromise between havinga short burst message length (fewer bits) and conveying meaningfulinformation (more bits) needed for a search and rescue operations,consistent with the desire to encrypt sensitive information which, ifintercepted by unintended listeners, would place the person beingrescued at greater risk. As used herein the words "bits" and theabbreviation of "bps" standing for "bits-per-second" are intended toinclude symbols and symbols-per-second, and "bits" and "ones" and"zeros" are intended to include binary or other logic states in anyform.

Comparing FIGS. 6 and 9, it will be seen that in protocol 200 of FIG. 9,ID portion 202 and UNIQUE WORD portion 210 have the same number of bitsas in FIG. 6, MESSAGE portion 206 has been shortened from 24 bits to 8bits, CHECK-SUM portion 208 has been shortened from 16 bits to 8 bitsand POSITION & TIME DATA portion 204 has been shortened from 64 to 52bits and accurate time information has been included. The BIT-SYNCportion (32 bits) at the beginning of the FIG. 6 message has beeneliminated. The combination of these modifications significantlyshortens the overall message length, for example, from 288 bits to 184bits, when half rate coded as shown in FIGS. 6 and 9. These messagelengths correspond to a transmission burst length reduction from about0.48 seconds to about 0.31 seconds, when transmitting at 600 bps andemploying half-rate coding as shown in the respective figures. Atransmission rate of 600 bps is preferred to fit within the typical fivekilohertz band-width of the present day satellite transmission channelsand provide robust communication. Higher or lower data rates can also beused, provided they are within the capability of the communicationchannel. The transmission burst length varies in direct proportion tothe data rate. For example, for a 184 bit signal, the transmission burstlength at data or symbol rates of 300, 600, 1200, 2400 and 4800 bpswould be about 0.61, 0.31, 0.15, 0.08 and 0.04 seconds, respectively.

FIG. 10 is similar to FIG. 9 but showing further details of message 200.When ID portion 202 has 16 bits, then 65,536 radios can be uniquelyidentified by means of ID portion 204 stored within memory 39 (see FIG.2). ID portion 202 provides authentication that a message received atmonitoring base station 26 or at rescue unit 28 has originated from aparticular known radio and is not a deception signal being sent byunfriendly forces.

When MESSAGE portion 206 has 8 bits, then any of 256 user selectablepre-stored messages represented by the eight bits can be included inmessage 200. As previously explained in connection FIG. 2, thesepre-stored messages can be loaded and altered by using fill connectorand external I/O 52. Thus, the messages pre-stored in radio 22 andrepresented by the MESSAGE bits, may be made specific to a particularmission on which radio 22 is to be carried or to a particular individualusing the radio or both.

POSITION & TIME DATA portion 204 is desirably divided into POSITION DATAportion 203 (e.g., 41 bits prior to half-rate encoding) and TIME DATAportion 205 (e.g., 11 bits prior to half-rate encoding). The informationcontained in TIME DATA portion 205 is derived, for example, fromaccurate time/frequency information provided via receiver 34 fromgeolocation source 30. In the preferred embodiment, TIME DATA portion205 includes accurate time information (e.g., at least hours andminutes). TIME DATA is conveniently used to provide CRYPTO-SYNC betweenradio 22 and base station 26 or rescue unit 28 for encryption anddecryption of the sensitive portions of message 200. For example, anencryption generator in radio 22 operating with a private key stored canbe used to generate a pseudo-random bit stream which is combined withthe sensitive data (e.g., ID, POSITION DATA, MESSAGE), e.g., byperforming an "Exclusive-OR" step, to provide the encrypted portion ofsignal 200. In order to decrypt such a message, the decryption processormust determine what portion of the pseudo-random bit stream was used forencryption. This is called achieving "CRYPTO-SYNC".

In some implementations, a known preamble message has often been used toachieve CRYPTO-SYNC. However, such preamble messages are not desirablewhen message length needs to be kept short. With the presentarrangement, the preamble can be omitted by including accurate timeinformation (TIME DATA) in message 200. The decryption processorrecovers plain-text from cipher-text by using this TIME DATA in a knownway to synchronize its own pseudo-random bit steam with what was usedfor encryption. This CRYPTO-SYNC information is provided bytransmitting, for example, accurate time corresponding to the initiationof the pseudo random code generator or corresponding to the particularportion of the pseudo-random bit stream used for encryption. Thus, TIMEDATA provides a useful CRYPTO-SYNC function in transmitter and receiverwithout need for any CRYPTO-SYNC preamble message. This reduces thenumber of bits in the message since fewer bits are required for TIMEDATA than would be required for an equally robust CRYPTO-SYNC preamble.

In the preferred embodiment, ID portion 202, POSITION DATA portion 203and MESSAGE portion 206 are encrypted prior to transmission, using TIMEDATA and an alterable private key. The alterable key is convenientlyloaded through port 52 and stored in memory 39 prior to use of radio 22for a particular mission or trip. Encryption may be performed using anyof a wide range of encryption methods of which DES, DVP and VINSON areknown, but non-limiting examples. Whether referred to generally asprivate-key or public-key systems, either the sender or receiver ofmessage 200 or both must possess the private (secret) key needed so thatthe enciphered portions of message 200 may be comprehensible to theintended recipient. As used herein, the words "private" or "secret" keyin connection with encryption or decryption are intended to encompassall such methods.

In the preferred embodiment, TIME DATA portion 205, CHECK-SUM portion208 and UNIQUE WORD portion 210 are not encrypted prior to transmission.TIME DATA remains unencrypted since, in the preferred embodiment, it isused to rapidly acquire CRYPTO-SYNC in the decryption process at thebase station. The CHECK-SUM is left unencrypted since it is used priorto decryption to determine whether the message received at base station26 or rescue unit 28 is the same message as was transmitted by radio 22,i.e., have message bits been lost or corrupted during transmission.UNIQUE WORD portion 210 is left unencrypted since it is used todetermine that a valid message has been received, irrespective of thecontent of the message, and irrespective of its origin. In the preferredembodiment of FIGS. 9-10, UNIQUE WORD portion 10 occupies last place inmessage 200.

While the use of TIME DATA is particularly convenient in connection withencryption and decryption, it is not essential and may be omitted. Forexample, if the encryption-decryption method chosen by a system designerdoes not require a CRYPTO-SYNC operation, then TIME DATA need not betransmitted for that purpose, and message 200 may be correspondinglyshortened. Persons of skill in the art will understand based on thedescription herein and the security requirements of their intendedapplication how to select an appropriate encryption-decryption methodand whether or not to include TIME DATA for CRYPTO-SYNC purposes.Omitting the 11 bits of half-rate coded TIME DATA included in theexemplary form of message 200 shown in FIG. 9, would shorten message 200by 22 bits, that is from 184 to 162 bits and the transmission time woulddrop from 0.31 to 0.27 seconds at 600 bps.

However, TIME DATA 205 may still be desirably transmitted for otherpurposes, as for example, to give the time and/or date of thegeolocation fix from receiver 34, 34' on which POSITION DATA 203 isbased. When TIME DATA is also transmitted for CRYPTO SYNCH purposes itcan serve both functions. When CRYPTO-SYNC is not required, fewer bitsmay be allocated to TIME DATA, the exact number depending upon theprecision with which one desires to known the age of the POSITION DATA.For example, if it is sufficient to know the age of the POSITION DATA to12 minutes within a 24 hour period, then only 7 bits are required,thereby shortening message 200 to 176 symbols and reducing thetransmission time at 600 bps to 0.29 seconds. The choice of whether ornot to include TIME DATA and with what resolution (how many bits) willdepend upon the particular needs of the intended users.

In the preferred embodiment, message 200 is tested after encryption andprior to transmission, to determine the number of logical 0-1 or 1-0transitions. If the number of such transitions is unusually small, thenit may be more difficult to rapidly and unequivocally acquire BIT-SYNCwhen signal 200 is received in receiving station 26, 28. To avoid thissituation, signal 200 is further processed in a predetermined mannerprior to transitions to increase the number of transitions withoutincreasing the signal length. The number of transitions can beincreased, for example, by subjecting the assembled message bit streamto an Exclusive-OR operation with a 101010 . . . (or other) bit stream,prior to being sent to transceiver 44, and testing to insure that theoutcome has more transitions. Several stored bit streams may be triedand the most effective in increasing the number of transitions used.Other methods known in the art for increasing the number of transitionswithout increasing the message length can also be used. This additionalstep also modifies the value of the UNIQUE WORD 210 in a predeterminedmanner. The modified value of UNIQUE WORD indicates to receiving station26, 28 that this additional processing has occurred and whatsupplementary bit stream was employed, so that it may be reversed priorto decryption or other signal processing. Persons of skill in the artwill understand based on the description herein when to include suchadditional processing step, depending upon the BIT-SYNC process beingused. The above-described operation is conveniently performed withinradio 22 using processor 38, 38' operating under control of a program orsub-routine stored, for example, in memory 39.

FIG. 11, analogous to FIG. 2, shows a further embodiment of radio 22,comprising transceiver 44 and associated antenna 21 for radiating andreceiving signals over links 23, 29, GPS receiver 34 and associatedantenna 33 for receiving geolocation and accurate time/frequencyinformation over link 32, receiver 34 being coupled by bus 36 to dataprocessor 38', clock 41 coupled to GPS receiver 34 by optional link 43and to data processor 38' by bus 42, memory 39 coupled to data processor38' by bus 40, fill connector and external I/O 52 coupled by bus 54' touser interface & display 48' which is in turn coupled to data processor38' by bus 49', power management unit 212 and battery 214 coupled toprocessor 38', user interface and display 48', GPS receiver 34 andtransceiver 44 by bus 216, and data buffer 220 coupled to data processor38' by data bus 222 and clock bus 224 and to transceiver 44 by signalpaths 60', 62'. Primed reference numbers in FIG. 11 are analogous tocorresponding unprimed reference numbers in FIG. 2.

The circuit of FIG. 11 differs from that of FIG. 2 in that the functionsperformed by crypto module 56 of FIG. 2 are now performed by processor38' (e.g., a type 68HC11 manufactured by Motorola, Inc. or other digitalsignal processor) using software, and data buffer 220 is employedbetween processor 38' and transceiver 44 so that message 200 maybeassembled and temporarily stored prior to transmission.

Processor 38' receives geolocation information and accuratetime/frequency information from GPS receiver 34 and user inputs frominterface 48' and, based on instructions and data stored in memory 39 orwithin processor 38', performs the encryption described in connectionwith FIG. 10, combines resulting cipher-text portions 202, 203 and 206with plain-text portions 205, 208 and 210 and transfers the result intodata buffer 220, desirably in the order indicated, but that is notessential. Data buffer 220, delivers message portions 202-210 totransceiver 44 over link 60' so that message 200 is burst transmittedwith message portions 202-210 desirably in the order shown in FIGS.9-10, that is with portions 202-208 preceding UNIQUE WORD portion 210.

An advantageous feature of radio 22 illustrated in FIG. 11, is theprovision of power manager 212 between battery 214 and the otherelements of radio 22 illustrated in FIG. 11. Power manager 212 desirablyoperates under the control of processor 38' to manage power delivery tovarious of elements 34, 38', 39, 41, 44, 48', 52 and 220 of radio 22during the process of preparing and sending message 200. Those of skillin the art will understand that the various components and elementsmaking up radio 22 require connections to battery 214, some of whichhave been omitted in FIG. 11 for convenience of explanation.

Power manager 212 disconnects power to any circuits not being currentlyused in the functioning of radio 22 to reduce power consumption frombattery 214. This extends the operating life of the radio betweenbattery charges. A further function of power manager 214 is to shut downduring encryption, all circuits (e.g., transceiver 44, user interfaceand display 48', fill connector and external I/O 52, etc.) except thoseessential to encryption (e.g., processor 38', memory 39, clock 41,etc.). This minimizes any plain-text or other signal leakage from radio22 which might be captured by an unfriendly listener and used to decryptor circumvent encryption of the sensitive portions of message 200. Thisprovides what is known in the art as TEMPEST protection. This is asignificant and valuable feature of the present invention.

FIG. 12 is a flow chart illustrating the preferred sequence 230 executedby radio 22 employing receiver 34', power manager 212 and processor 38'in preparing message 200. In step 232, receiver 34' receives GPSposition and time data from down-link 32 and in step 234 transfers it toprocessor 38'. In step 236, processor 38' and power manager 212 shutdown, preferably, all other circuits in radio 324 not needed for theoperation of processor 38' to perform the desired encryption steps togenerate cipher-text message portions 202, 203 and 206 (and any othersthat might be desired). For example, one or both of transceiver 44 andreceiver 34 are powered down. Similarly, other elements of radio 22which are not essential to the encryption process are also desirablyshut down, as for example, fill connector and external I/O 52 (after thekey is loaded) and user interface & display 48' (after the user hasinput any predetermined message choices). Data buffer 220 may also bedisabled until encryption is complete, but this is not essential.

Encryption using TIME DATA and the alterable secret key, is performed instep 238 and the resulting cipher-text portions of message 200 sent todata buffer 220 along with the plain-text portions of message 200 instep 240. The encryption process is very brief, for example, of theorder of a millisecond or less. Once encryption is complete, powermanager 212 restores power to the other elements of radio 22, preferablyin the order in which they are needed in order to transmit message 200.Thus, buffer 220 (if inactive) and transceiver 44 would be promptlyre-energized so that burst transmission can occur. MSK (e.g., CPFSK) isa convenient modulation technique for message 200.

Receiver 34 is re-energized, for example, upon operator command oraccording to a predetermined schedule, depending upon how often positionand time information or user inputs are needed. Interface and display48' are energized to allow the user to see that an acknowledgmentmessage has been received from base station 26 or rescue vehicle 28.Ordinarily, fill connector and external I/O 52 need not be energizeduntil new data or a new key need to be loaded into radio 22. Bydeactivating all non-essential functions during encryption, undesired(TEMPEST) emissions that could compromise the encrypted message areminimized and battery life is extended.

FIG. 13 is a simplified flow chart illustrating how highly abbreviatedmessage 200 is handled in base station 26 (or equivalently in rescuevehicle 28). Base station 26 receives signal 250 which comprises a noiseportion 252, a subsequent coherent portion 254 which may or may not be asignal from radio 22, and a further noise portion 256. Referring now toFIGS. 4 and 13, transceiver 144 and channelized receiver and signalprocessor 146 detect signal 250 including coherent portion 254, and passthe resulting information to data processor 152. The receivedinformation is processed and temporarily stored. This is convenientlyaccomplished, for example, by digitizing and storing the IF signalextracted from receiver 146. In the preferred embodiment, the IF signalhas a 10 kHz center frequency and 3 kHz bandwidth and is readily sampledand digitized using techniques well known in the art. The digitizedinformation may be stored in any convenient location in base station 26,for example, in receiver-processor 146 or 152 or computer system 154. Inthe course of performing detection and analysis, the bit timing ofcoherent portion 254 is determined, that is, when the bits occur intime, how many there are and which are one's and zero's (or any othersymbol or logical representation being used). With bit timingestablished, coherent portion 254 is tested to determine whether itcontains UNIQUE WORD 210, a copy of which is stored or regenerated inbase station 26 (or rescue vehicle 28). If a match is found, thencoherent signal portion 254 is known to correspond to message 200.

Once UNIQUE WORD 210 is detected, the digitized information comprisingcoherent message portion 254 is retrieved from memory. In the preferredembodiment this is demodulated using the recovered carrier and BIT-SYNCtiming information already obtained. The cipher-text portions are thenseparated from the plain-text portions and the cipher-text decryptedusing TIME DATA (if needed) and the appropriate key stored in basestation 26 (or rescue vehicle 28) corresponding to the alterable keypreviously loaded into radio 22 and used for encryption. The decryptionkey may be the same key in a single key system or a mating key in abinary key system. In either case, the purpose of the alterable key(s)is to render the cipher-text incomprehensible to unintended recipients,but comprehensible to the intended recipients. The data retrieved frommessage 200, is then displayed using computer system 154 or transmittedto other units, as for example, to rescue vehicle 28. A "messagereceived" (ACK) response is desirably transmitted back to radio 22, ashas been previously described.

The above-described process carried out in station 26, 28 receivingsignal 250 is a two-pass signal recognition process. It need not occurin real time. On the first pass through the detection and analysis chainof the receiving station, the coherent carrier is acquired using meanswell known in the art, the bit stream is detected, the bit timing isrecovered from the coherent signal, the bit stream checked for theUNIQUE WORD, and a reject or reprocess decision made. In the preferredembodiment described in connection with FIGS. 9-10, the UNIQUE WORDcomprises the last 16 bits of the message. The receiving stationdetection and analysis chain is desirably preprogrammed to sequentially(or concurrently) compare 16 bit blocks of the detected bit steam (e.g.,bits 1-16, 2-17, 3-18, etc.) with the stored UNIQUE WORD to detect amatch. If no match is found, the received signal does not have a portionwhich corresponds to message 200 and the signal is rejected. When aUNIQUE WORD match is found, then the time segment of the storeddigitized signals which corresponds to message 200 is known and at leastthis part of the received signal is retained in memory for furtherprocessing using the carrier and BIT-SYNC information developed in thefirst pass. In a second pass, the relevant portion of the storeddigitized information is further processed to verify the CHECK-SUMportion, to recover the TIME DATA portion (if any) and to pass theencrypted portions through decryption to extract the ID portion, thePOSITION DATA portion and any MESSAGE portion.

The above-described procedure has the advantage that no BIT-SYNC leaderor other preamble is needed for the message from radio 22. Message 200serves as its own BIT-SYNC. While the UNIQUE WORD portion can be placedanywhere in message 200, it is preferably placed at the end of message200, so that bit timing may be recovered before the UNIQUE WORD portionof the message is reached. In view of the very few bits in the totalmessage, placing the UNIQUE WORD at the end improves the likelihood ofBIT SYNC being achieved before the UNIQUE WORD arrives. This increasesthe robustness of the communication process and improves the probabilitythat a message from rescue radio 22 will be identified and decrypted thefirst time received. Because repeated transmissions of a signal fromrescue radio 22 may place the user at substantially greater risk when inthe presence of unfriendly forces, minimizing the number oftransmissions is an important advantage of the present invention.

FIG. 14, which is analogous to FIG. 7, shows the preferred protocol formessage 260 sent from the monitoring base station 26 or rescue vehicle28 to radio 22 to acknowledge receipt of the message described inconnection with FIGS. 9-10. The same message is also repetitivelytransmitted by base station 26 via satellite 24 to act as an antennaalignment signal for mobile radio 22. Message 260 comprises BIT SYNCportion 262 (e.g., 32 bits), UNIQUE WORD portion 210' (e.g., 16 bits),ID portion 202' (e.g., 16 bits) and MESSAGE portion 206' (e.g., 8 bits).When ID portion 202' and MESSAGE portion 206' are half-rate coded asindicated in FIG. 14, then message 260 has a preferred bit length of 96bits, i.e., 52% of the preferred bit length (184 bits) of message 200.

The 32 bit BIT SYNC portion 262 of message 260 allows hand-held radio 22to easily bit synchronize with the received signal, 16-bit UNIQUE WORDportion 210' identifies the start of the return message, and 8-bitMESSAGE portion 206' identifies one of the 256 text messages pre-storedin radio 22 which may be placed on display 48 or otherwise enunciated.ID portion 202' and MESSAGE portion 210' may be a null set or omittedwhen signal 260 is being provided prior to reception of signal 200 forantenna pointing purposes. When 16-bit ID portion 202 in message 200 wassent by radio 22 to monitoring station 26, 28 in encrypted form, only anauthorized recipient possessing the necessary variable key could decryptit and re transmit the correct ID portion 202' back to radio 22 as apart of return message 260. In the preferred embodiment, ID portion 202'may be the same as the unencrypted ID portion stored within radio 22 ormay be a different but related ID portion.

For example, a particular radio 22 may have stored internally aparticular value (X) for ID portion 202 which it uses to identify itselfin its messages sent to monitoring stations 26, 28, and may also havestored internally, a different value (Y) for ID portion 202' which ituses to identify messages intended for it. Thus, the ID portions in theoutgoing and incoming messages need not be the same. In a like manner,UNIQUE WORD portion 210' may be the same or different than UNIQUE WORDportion 210. Further, the number of different ID portions or differentUNIQUE WORD portions used by a radio need not be limited merely to oneor two values. Radio 22 and monitoring stations 26, 28 may be equippedwith tables of random values to be used in successive communicationsback and forth, i.e., the ID and/or UNIQUE WORD values may be changed ina known way for each transmission. This arrangement decreases the numberof unequivocal radio ID numbers and/or UNIQUE WORDS available for use bydifferent radios at the same time. However, because each message 200,260 contains both UNIQUE WORD portion 210 (e.g., 216 possible values)and ID portion 202 (e.g., 216 possible values), the total number ofradios 22 that may be uniquely identified is very large (e.g., 2¹⁶×2¹⁶).

By considering the ID and the UNIQUE WORD together as a radioidentifier, and permitting use of multiple valid UNIQUE WORDS, the totalnumber of bits devoted to this task may be further reduced. For example,by allowing a comparatively small number (e.g., 2⁸) of UNIQUE WORDS tobe valid, the number of ID bits can be reduced (e.g., from 16 to 8bits). Because the ID bits are being encoded at rate 1/2. reducing theID bits from 16 to 8, shortens the message by twice the reduction in IDbits. Thus, if this change alone is made, the number of message bitswould be reduce from 184 to 168 and the burst length at 600 bps woulddrop from 0.31 to 0.28 seconds. If this change is combined witheliminating TIME DATA then the message is shortened by 184-16-11=157bits and transmission time to 157/600=0.26 seconds. Having a largenumber of permitted UNIQUE WORDS is not desirable since it increases thebase station processing time and the number of false positives, i.e.,random bit sequences which appear (incorrectly) to correspond to a realmessage. The preferred embodiment illustrated herein is a desirablecompromise between these trade-offs.

Message 260 may be entirely in plain-text or may be partly encrypted,for example, in the same manner as was used in encrypting the portionsof messages 200 generated by radio 22 and using the appropriate variablekey. BIT SYNC portion 262 and UNIQUE WORD portion 210' are desirably inplain text and one or both of ID portion 202' and MESSAGE portion 206'are encrypted by station 26, 28 prior to transmission to radio 22 anddecrypted on receipt by radio 22. In the preferred embodiment, IDportion 202' is left unencrypted so that radio 22 may quickly identifymessages intended for it without the delay associated with a decryptionprocess.

It was found that when the burst messages were shortened in the mannerdescribed above there was not a significant degradation of the quality(e.g., detectability and accuracy) and value (e.g., useful content) ofthe information conveyed, that is, an equally useful message with theabout same probability of receipt and decryption by the monitoringstation is conveyed in one-third less transmit time. When the personbeing rescued is exposed to unfriendly forces, minimizing the transmittime and therefore the opportunity for an unfriendly force totriangulate on the transmitter, is extremely important. Hence, thisimprovement has great utility and is highly desirable.

A further advantage resulting from the shorter burst transmission timefrom the hand-held emergency radio to the monitoring base station, inthat the relative proportion of time that the base station istransmitting an antenna alignment signal to the hand held unit may beincreased. This means that the hand held unit transmitting antenna maybe more quickly and easily aligned to communicate with the satellitecommunication channel. Alignment of the transceiver antenna of thehand-held unit so as to have the maximum probability of communicatingwith the satellite is very desirable if the transmitter power andbattery size in the hand-held unit are to be minimized. Minimizing thetransmitter power is important to reduce the probability of interceptionby unfriendly forces and to reduce the energy drain from the battery.Minimizing battery drain is important to achieving a small size unitwhich still has sufficient energy reserves in the battery to allow foradequate talk (and listen) time.

FIG. 15 illustrates an arrangement in which base station 26 (see FIG. 1)transmits through satellite 24, antenna pointing signal 260 of duration211. Base station 26 then shuts off its transmitter and energizes itsreceiver for time period 280 to listen to signals from emergencytransceivers 22. The sum of base station transmit interval 261 and basestation receive interval 280 define overall communication repeatinterval 282. The relative length of time intervals 261 and 280 (or themagnitude of interval 282) depend upon duration 201 of signal 200 (seeFIGS. 9-10) from hand-held unit 22 and duration 261 of signal 260 frombase station 26 or rescue unit 28 (see FIG. 14). Guard band times 283 ofduration 284 are provided before and after the nominal transmission timeof emergency signal 200 within base station listening interval 280 toavoid losing part of signal 200 due to variations in the propagationdelay. For example, if processor 38, 38' commands transceiver 44 (seeFIGS. 2 and 11) to transmit burst signal 200 after the end of antennapointing signal 260 received from base station 26, signal 200 willarrive at base station 26 approximately two propagation delays afterbase station 26 stopped transmitting. Thus, guard bands 283 should be atleast equal to twice the longest expected propagation delay variationbetween radio 22 and base station 26. While guard bands 283 areillustrated as being of equal length 284, those of skill in the are willunderstand based on the teachings herein that this is merely forconvenience of explanation and that they may be of unequal or variablelength, depending upon the particular characteristics of the systembeing used.

In the preferred embodiment, using half-rate coding for the ID portionand MESSAGE portion as indicated in FIG. 14, 96 bits or symbols arerequired for message 260. Signal 260 is preferably sent as an FSK signalat a rate of about 150 bps, but lower or higher rates may be used. Usinga lower bit rate for signal 260 as compared to signal 200 allows theshort (96 bit) outgoing signal 260 from base station 26 (or rescuevehicle 28) to be relatively more spread out in time (96 bits/150bps=640 milliseconds) as compared to signal 200. The low bit rate andFSK modulation provides a more robust communication capability andallows for a very simple receiver in radio 22. This improves theprobability that radio 22 will receive signal 260 even under adverseconditions and reduces the overall size and cost or radio 22.

Signal 260 transmitted from the base station to radio 22 is used by theoperator of radio 22 to orient antenna 21 of radio 22 to improve theprobability that when radio 22 transmits message 200, it will bereceived by satellite 24 and passed to base station 26, or similarly forrescue vehicle 28. Thus, it is desirable that signal 260 occupy as largea fraction of total signal interval 282 as possible without having anunduly long signal repeat interval.

For example, the signaling arrangements illustrated in FIGS. 6 and 7showed an "outgoing" (base 26, 28 to mobile 22) transmission time of 270milliseconds, and an "incoming" (mobile 22 to base 26, 28) transmissiontime of 480 milliseconds. Assuming guardband intervals of 30milliseconds on either side of the incoming (or outgoing) signal, thetotal signal repeat interval time (outgoing+guardband+incoming+guardbandtime) equals 270+30+480+30=910 milliseconds, of which the outgoingsignal time occupied only 33%. While the overall signal repeat time isfrequent enough, the comparatively small percentage occupied by theoutgoing signal (used for antenna pointing) is a disadvantage.

In the embodiment illustrated in FIGS. 9, 14 and 15, outgoing signal 260has duration 261 of (96 bits)/(150 bps)=640 milliseconds, and incomingsignal 200 has duration 201 of (184 bits)/(600 bps)=307 milliseconds.Including 30 millisecond guard band durations 284, gives an overallsignal repeat time of 640 30+307+30=1007 milliseconds, of which outgoingsignal duration 261 occupies about 64%. Thus, a much larger proportionof the signal repeat time is provided for signal 260 from base station26 or rescue vehicle 28 to facilitate antenna pointing by the user ofradio 22. In addition, the incoming signal time has been reduced by 36%to 307 milliseconds. Radio 22 can time its transmission to fall withinthe 367 millisecond gap 280 between successive transmission of signal260 second gap by noting the termination of signals 260 and thentransmitting signal 200. Alternatively, high accuracy time informationderived from GPS receiver 34' may be used to determine an accurate clocktime when signal 200 can be sent without interfering with signal 260. Ina preferred embodiment, about a 30 millisecond guard band is providedbefore and after transmission of signal 200. While the foregoingdiscussion has been made in terms of communicating from base station 26via satellite 24 to and from mobile radio 22, this description alsoapplies to transmissions between rescue unit 28 and mobile radio 22.

It is desirable that display 48, 48' show the received signal strengthof outbound message 260 from base station 26 so as to facilitate theuser being able to orient antenna 21 of radio 22 to achieve maximumcoupling to satellite 24 (or rescue vehicle 28). In the preferredembodiment as illustrated in FIGS. 1 and 11, antenna 21 is a simpleblade or whip antenna, preferably of a folding or retracting variety.The typical umbrella type antenna conventionally used for satellitecommunications is not required. Since the satellite generally cannot beseen by naked eye, having an indication when a favorable antennaorientation is achieved is highly desirable. Once radio 22 is properlyoriented to as to achieve good antenna coupling to the satellite, thelink margin is more than sufficient for the described purpose and robustcommunication may be achieved.

Based on the foregoing description, it will be apparent to those ofskill in the art that the present invention solves the problems andachieves the goals set forth earlier, and has substantial advantages aspointed out herein, namely, it provides a secure mobile radio welladapted to use as an emergency radio by means of a built-in geolocationand timing receiver which provides accurate local position and time. Theaccurate local position is included automatically in the message sentout by the mobile radio operating in its emergency mode. The positioninformation and messages are encrypted so that unfriendly listenerscannot locate the mobile radio or determine its status from thetransmitted information.

The invented radio further provides Doppler shift correction of itstransmitted signals by listening to signals transmitted through thesatellite communication system to which it is tuned and by determiningthe frequency of received signals using accurate time/frequencyinformation proved by the on-board geolocation receiver, and bycomparing the received frequencies to predetermined anticipatedfrequencies and then shifting the frequency of its internal transmitterin anticipation of the Doppler shift that will be encountered by itstransmitted signal as it makes its way back to the communication systembase station so that the signal received at the base station issubstantially Doppler free. This reduces base-station signal acquisitiontime. By listening to transmission from radio 22, the same procedure maybe used by base station 26 to correct for the Doppler shift of signalsit intends to transmit to radio 22, so as to reduce signal acquisitiontime in radio 22. A further feature of the invented radio is that onlyvery short message bursts are required, generally only a fraction of asecond, thereby reducing the risk of radio 22 being located byunfriendly forces. In addition, the transmitted signals may contain aunique address so that the mobile unit can uniquely identify signalsdirected to it from other signals on the same channel.

While the present invention has been described in terms of particulararrangement, satellites, communication systems, geolocation systems andsteps, and these choices are for convenience of explanation and notintended to be limiting and, as those of skill in the art willunderstand based on the description herein, the present inventionapplies to other arrangements, satellites, communication systems,geolocation systems, and steps, and it is intended to include in theclaims that follow, these and other variations as will occur to those ofskill in the art based on the present disclosure.

We claim:
 1. A transceiver comprising:a first radio receiver and asecond radio receiver, a radio transmitter and an antenna supported by acommon housing, wherein said first radio receiver receives first signalsfrom a first geolocation means for determining location information andhigh accuracy clocking information, wherein said location informationdescribes a location of said transceiver; clock means, supported by saidcommon housing and coupled to said first radio receiver, said clockmeans for providing timing and frequency control signals, said clockmeans slaved to said high accuracy clocking information; wherein saidradio transmitter is coupled to said antenna and receives timing andfrequency control signals from said clock means, said radio transmitterfor sending another signal including said transceiver locationinformation to a communication system, and wherein said second radioreceiver receives second signals from said communication system.
 2. Atransceiver as claimed in claim 1, wherein said another signal is aburst transmission signal having a duration of less than a second at abit rate of 600 bps.
 3. A transceiver as claimed in claim 2, whereinsaid burst transmission signal has a duration of less than a half asecond.
 4. A transceiver as claimed in claim 1, wherein said anothersignal includes predefined user selectable status messages identified byone or two alpha-numeric characters.
 5. A transceiver as claimed inclaim 1, wherein said another signal comprises, at least during a firsttransmission thereof, a unique word portion, an identification portion,a position data portion, a message portion and an error detectionportion.
 6. A transceiver as claimed in claim 1, further comprisingmeans for detecting a portion of said second signals indicating that abase station coupled to said communication system has received saidlocation information transmitted by said transceiver.
 7. A transceiveras claimed in claim 6, further comprising indicator means for indicatingreceipt of said portion of said second signals indicating that said basestation has received said location information transmitted by saidtransceiver.
 8. A radio comprising:receiver means responsive to GPSsatellites for receiving therefrom information permitting said radio todetermine local position information describing a location of said radioand accurate time information; transceiver means for communicating withat least one remote station via a transceiver satellite, by sending aburst signal of less than one second duration including said localposition information to said at least one remote station via saidtransceiver satellite and receiving therefrom an acknowledgment thatsaid local position information has been received from said at least oneremote station, wherein said transceiver means uses said accurate timeinformation from said receiver means to determine a narrow time windowduring which to transmit said burst signal to said at least one remotestation; one or more antennas coupled to said receiver means and saidtransceiver means; and a common hand-portable housing containing saidreceiver means and transceiver means and supporting said one or moreantennas.
 9. A radio as claimed in claim 8, wherein said burst signalcontains 288 or fewer bits.
 10. A radio as claimed in claim 8, whereinsaid burst signal comprises half-rate coded information at a serial bitrate of about 600 bits per second.
 11. A radio as claimed in claim 8,wherein said burst signal contains redundant information bits.
 12. Aradio as claimed in claim 8, wherein said burst signal has a duration ofless than one-half second.
 13. A radio as claimed in claim 8, whereinsaid burst signal comprises, at least during a first transmissionthereof, a unique word portion, an identification portion, a positiondata portion, a message portion and an error detection portion.
 14. Adevice comprising:a geolocation receiver for providing, from geolocationmeans, local position information describing a location of said deviceand high accuracy clocking information; a transceiver coupled to saidgeolocation receiver for sending a message including said local positioninformation and one or more symbols indicative of user status to acommunication system, said transceiver using said high accuracy clockinginformation provided by said geolocation receiver to determine a narrowtime window during which to send said message to said communicationsystem; input means for receiving user status information; processormeans coupled to said input means and said transceiver, said processormeans for receiving said user status information from said input meansand providing to said transceiver means for inclusion in said message,said one or more symbols which correspond to said user statusinformation; at least one antenna coupled to said geolocation receiverand said transceiver; and a common housing containing said geolocationreceiver, transceiver, input means and processor means and coupled tosaid at least one antenna.
 15. A device as claimed in claim 14, whereinsaid message is a burst message of less than one second duration at abit rate of about 600 bps.
 16. A device as claimed in claim 15, whereinsaid message is a burst message of less than one-half second duration.17. A device as claimed in claim 14, wherein said message has atransmission rate of not more than 600 bits per second and a duration ofless than about one-half second.
 18. A radio, said radio being aposition locating radio and comprising:a receiver for providing localposition information and high accuracy time or frequency information; aclock coupled to said receiver and having an output corrected by saidhigh accuracy time or frequency information from said receiver; atransceiver for communicating with at least one remote station through acommunication system, said transceiver coupled to said clock andreceiving high accuracy clocking signals therefrom; input means forreceiving from a user of said radio, user status information describinga status of said user; and processing means coupled to said transceiverand said input means, said processing means for receiving said userstatus information from said input means, selecting one or more symbolshaving a predetermined meaning most closely corresponding to said userstatus information from a predetermined list, and providing saidselected symbols to said transceiver; wherein said receiver, clock,transceiver, input means, and processing means are carried by a commonman-portable housing supporting one or more antennas coupled to saidreceiver and said transceiver.
 19. A radio as claimed in claim 18,wherein said transceiver is a burst transceiver providing signal burstscontaining said local position information in redundant form.
 20. Aradio as claimed in claim 19, wherein said signal bursts have a timeduration of less than about one-half second when transmitted at a bitrate of about 600 bits per second.
 21. A transceiver comprising:a firstradio receiver for receiving first signals from a first geolocationmeans for determining therefrom transceiver location information andhigh accuracy time and frequency information; a clock circuit coupled tosaid first radio receiver, said clock circuit for providing timing andfrequency control signals, said clock circuit slaved to said highaccuracy time and frequency information from said first radio receiver;a transmitter coupled to said first radio receiver and said clockcircuit, said transmitter sending another signal containing saidtransceiver location information to a communication system; and a secondradio receiver for receiving second signals from said communicationsystem, wherein said first and second radio receivers, said transmitter,and said clock circuit are located in a common hand-holdable housing andcoupled to one or more antennas supported by said common hand-holdablehousing.
 22. A transceiver as claimed in claim 21, wherein said anothersignal is a burst transmission signal having a duration of less than onesecond.
 23. A transceiver as claimed in claim 22, wherein said bursttransmission signal has a duration of less than one-half of one second.24. A transceiver as claimed in claim 21, wherein said another signalfurther contains predefined user status messages identified by one ortwo symbols.
 25. A transceiver as claimed in claim 21, wherein saidanother signal comprises, at least during a first transmission thereof,a unique word portion, an identification portion, a position dataportion, a message portion and an error detection portion in apredetermined order.
 26. A transceiver as claimed in claim 21, furthercomprising means for detecting a portion of said second signalindicating that a base station coupled to said communication system hasreceived said encrypted transceiver location information transmitted bysaid transmitter.
 27. A transceiver as claimed in claim 26, furthercomprising indicator means for indicating receipt of said portion ofsaid second signal indicating that said base station has received saidtransceiver location information transmitted by said transmitter.